_________we sleep together or apart is one of the biggest cultural differences, and only in western societies ______ the feeling that babies should sleep apart from their parents.

When exposed to stress, in ________ form, all of us react both chemically and physically.

In the time of HINI, scientists ____ their spare time to conduct research on the ways of curing the disease.

I would have come earlier，but I________that you were waiting for me.[ ]

Listen! Some birds aresinging __________ the tree.